I think they are saying you could 'roll back' on your own node to a point where you did have coin age, and try to attack from that point.
But it's not clear how you would get very far because there would be a longer chain soon and your coin age would be used up fast.
That is like saying you can't 51% attack a PoW network because the main chain is growing. Yeah if you have less computing power than the "good guys" you can't but what if you have more? With more than half of the network computing power you will eventually build the longest chain. With a PoS you will eventually build the longest chain if you have more than half of the network stake.
True. In both cases a majority ownership either in stake or hashing power would make an attack possible.
But how does that make PoS inferior ?